There are a number of issues associated with radiation-cured inkjet fluids which make them particularly prone to generating high levels of migratable species. Firstly, the low viscosity of these fluids increases the effect of oxygen inhibition requiring the use of relatively high concentrations of photoinitiators, sometimes up to 10% (w/w), or greater, of the total ink formulation. Thus, the risk of contamination from photoinitiators and their photodecomposition side-products is increased. Secondly, the low viscosity requirement of inkjet fluids restricts the use of highly-functional and/or high molecular weight monomers and oligomers (which tend to have high viscosities) which are available for use in UV flexo, gravure and offset inks. The need to use mainly difunctional (and perhaps monofunctional) monomers, with only a limited amount of tri-, and higher, functional monomers and oligomers also increases the risk of unwanted migration from uncured material. Thirdly, inkjet fluids are applied at higher film weights than other traditional printing methods (around 8-12 μm). This also increases the risk associated with migration.
There are a number of patents describing polyethylene imine (PEI) as a component to reduce odors arising from aldehydes, acids, etc., but no teachings for using PEI (or other amines, especially polymer bound amines) in an overprint varnish to reduce migratables from underlying substrates or cured finished inks films, especially UV-curable finished ink films.
For instance, Milliken (US 2008/0164439A1) describes a wash-durable fabric treatment which is effective in eliminating chemicals associated with body odor. The basis of this treatment is an esterified PEI, which when applied to fabric was shown to reduce the level of isovaleric acid and isobutyraldehyde, even after 20 washes.
Aikoh Co. (U.S. Pat. No. 4,938,957) describes a deodorant composition comprising PEI, or an alkoxylated derivative of PEI, which was effective in reducing acetaldehyde and hydrogen sulfide levels.
Similarly, U.S. Pat. No. 4,941,991 describes a deodorant composition that could be applied to air filters to reduce the level of malodorous air-borne agents.
Xerox (U.S. Pat. No. 5,017,644) describes an (aqueous) inkjet composition containing an N-hydroxy substituted PEI. The PEI was used as a vehicle in the described ink composition with one of the mentioned advantages being an improvement to the open time. However, there is no mention of the odor-reducing capacity of this ink, nor is there any teaching of its use as an overprint varnish.
In terms of Inkjet fluids having low migration potential, WO2009/053348A1 describes UV-curable fluids which produce low levels of migratables upon curing. It also describes low migration radiation-curable inkjet fluids and reveals the use of a vinylether containing monomer in the preparation of radiation-curable inkjet fluids for low migration applications.
Accordingly, the prior art fails to teach an overprint coating or varnish (OPV) which can absorb the deleterious migratables emanating from a UV-cured ink or coating.